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Trying to understand why the dinosaurs became extinct has become one of the great geological detective stories. Some recent findings from the small Mexican village of Chicxulub have given scientists new hope that the answer may soon be known. The story starts a little over 15 years ago in the town of Gubbio, Italy, where geologist Walter Alvarez was collecting sediment from a layer of rock which marked the boundary between the Cretaceous and Tertiary time periods. Geologists had long known that this boundary was important because it marked a period in the Earth's history, some 65 million years ago, when almost half of all known species suddenly disappeared, including the dinosaurs. Walter brought some of his sample back to the United States and his father, Nobel prize-winning physicist Luis Alvarez, analyzed it for any unusual chemicals. To their surprise, the sample showed a high concentration of the element iridium, a substance rare on Earth but common in meteorites. To make sure there was nothing unusual about the Gubbio sample, they analyzed other K-T boundary strata from around the world. They found extra iridium in these samples as well. Using the average thickness of the clay as a guide, they calculated a meteorite would require a diameter of about 10 kilometers (6 miles) to produce this much iridium. If a meteorite that size had hit Earth, the results could explain the extinction of dinosaurs. The dust thrown up in the air would have caused major climatic changes to which many animals could not rapidly adapt. A major problem with this theory, however, was that a 10-kilometer meteorite would leave a very large crater, between 150 and 200 kilometers (93-124 miles) in diameter. While Earth has many impact craters on the surface, few are even close to this size. Because 65 million years had passed since the hypothetical impact, scientists decided to shift the search underground. A crater that old would almost certainly have been filled in. Just by chance, a Mexican oil company drilling off the coast of Yucatan discovered what appeared to be a crater about one kilometer (0.6 miles) under the surface near the village of Chicxulub. When core samples were analyzed, they showed the crater to be about 180 kilometers (112 miles) in diameter and 65 million years old. Was this the cause of the dinosaurs' extinction? The jury is still out, but evidence strongly suggests that the case of the disappearing dinosaurs may finally be solved. Suppose a ten-kilometer meteorite hit Earth today. What effect would it have on humans? Massive meteorite impacts are only one possible cause of climatic change. Volcanoes, forest fires, and industrial pollutants can also affect weather. Many scientists feel that human activities may cause climatic change. What do you think? How have meteor impacts affected our moon, Mars, and other solar system members? What did astronomers learn when they watched fragments of the Shoemaker-Levy comet hit Jupiter in July 1994?